1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus.
2. Description of the Related Art
There is an image forming apparatus in which a sheet discriminating device such as an optical sensor is provided for automatically discriminating the type of a sheet (see Japanese Patent Application Laid-Open No. 2008-94600). A technology for controlling development conditions, transfer conditions, fixing conditions, or the like according to the discriminated type of the sheet is proposed (see Japanese Patent Application Laid-Open No. 2006-175611).
Further, conventionally, in a color laser printer, for example, single-color tone patches of yellow (Y), magenta (M), cyan (C), and black (K) or patches of colors made by mixing C, M, and Y are formed on a sheet. Further, density or chromaticity of the patches on the sheet is detected by an optical sensor after the patches are fixed. Further, there is proposed a technology for controlling the density or chromaticity of an ultimate output image which is formed on the sheet by feeding the result of the detection back to a calibration table for calibrating an amount of exposure, process conditions, and density-tone characteristics of the image forming portion (see Japanese Patent Application Laid-Open No. 2006-143420).
Still further, conventionally, in a sheet feeding apparatus for supplying a sheet to an image forming apparatus, an ultrasonic generator and an ultrasonic receiver are provided so as to be opposed to each other and so as to sandwich therebetween a sheet conveying path through which the sheet passes such that whether a double feeding occurs or not is detected based on the amount of attenuation of the ultrasonic wave. Further, there is proposed a technology for optimizing the rotation speed of a sheet feeding roller according to the result of the detection (see Japanese Patent Application Laid-Open No. 2008-189449).
In a conventional image forming apparatus in which the type of a sheet and the double feeding of sheets are detected using those technologies, in order to perform appropriate control, it is necessary to improve the detection accuracy of the detecting means such as a sensor. When, for example, a characteristic of the sheet such as the type of the sheet is detected by the detecting means, if the sheet flutters at a detecting position of the detecting means, the detection accuracy is reduced. In this context, in order to suppress a flutter of the sheet at the detecting position of the detecting means, a pressure-contacting means may bring the sheet into pressure contact with a guide provided in the vicinity of the detecting means to thereby improve the detection accuracy of the detecting means. However, if such a structure is adopted, a sheet which is conveyed is brought into pressure contact with the guide at all times even when the detecting means does not carry out detection, that is, even when the prevention of the flutter of the sheet is not required. In this case, in the context of recent demand for higher durability, there is apprehension that the guide is abraded by sheets because, when a sheet is conveyed, the sheet slides while the whole sheet from its leading edge to its trailing edge is in pressure contact with the guide. If the guide is abraded in this way, the amount of abrasion of the guide increases as the number of sheets which passes increases, and there is a fear in that, ultimately, the position of the sheet is away from the focal position of the sensor or the flutter of the sheet cannot be suppressed. As a result, the detection accuracy of the detecting means is reduced.